Polypropylene resins are relatively inexpensive and excellent in various physical properties, and thus are used in a variety of molded articles today. They are used in a wide range of fields, for example, including (1) injection molded articles such as caps, (2) extrusion molded articles such as sheets, and (3) blow molded articles such as ducts.
A variety of polypropylene resins are employed according to characteristics required for respective product applications and molding applications. The examples include (1) propylene homopolymers, (2) copolymers of propylene and α-olefins (particularly ethylene), and (3) blends of these (1) and (2).
On the other hand, means for meeting characteristics required for product applications and molding applications include (1) a method of altering various characteristics of polypropylene (polymer, copolymer, or a blend thereof) as mentioned above and (2) means by use of formulation ingredients (including a modifying agent and an additive with specific properties).
The present inventor has studied a propylene resin composition that is excellent in rigidity, impact resistance and blow molding properties as well as drop impact properties at low temperatures, and a blow molded panel comprised of the composition.
Polypropylene resin is frequently used for the flow molded panel as mentioned above because it satisfies rigidity and impact resistance required as a structure member and can be blow molded.
The blow molded panel as mentioned above is excellent in cold resistance impact strength as compared with a homopolymer and a random copolymer. A field requiring impact strength at −30° C. as well as heat resistance at 100° C. such as automobile parts uses a block copolymer produced by block copolymerizing 5 to 10 weight % of ethylene with propylene (see “Plastics Age” 2. Polypropylene, II. Molding Technique and Applications of Various Materials, Blow Molding Technique, published by Plastics Age Co., Ltd., October, 1984).
It is known that high density polyethylene is formulated with a polypropylene block copolymer or polypropylene homopolymer in order to improve blow molding properties (see Japanese Patent Application Publication (JP-B) Nos. 6-13625 and 6-80140).
JP-B-6-13625 describes a resin composition for blow molding in which talc is formulated for restraining the gloss of polypropylene resin (homopolymer or block copolymer) to obtain low gloss properties (surface deglazing) and high density polyethylene exhibiting a melt index within a specific range is formulated in order to improve gloss unevenness during talc formulation. Descriptions in JP-B-6-13625 are given from the viewpoint of obtaining low gloss properties with no gloss unevenness without describing what kind of high density polyethylene prevents blow molding properties. JP-B-6-13625 does not describe the improvement of propylene resin itself with blow molding properties or the improvement of high density polyethylene and blow molding properties or the improvement of impact resistance.
JP-B-6-80140 specifies the relationship of melt flow rates between a polypropylene block copolymer and high density polyethylene to be formulated therewith in order to improve gloss properties of the polypropylene block copolymer. The prior art of JP-B-6-80140 describes that a rubber material such as polyethylene, polyisobutylene or polybutadiene is sometimes added as means for improving the impact strength of polypropylene, particularly impact resistance at low temperatures, and that the addition generates a decrease in rigidity. Example 1 in JP-B-6-80140 describes that the drop impact strength at −30° C. is 18 kg/cm. However, test strips as a measurement target are injection molded articles, and the whole specification does not refer to blow molding properties.
Japanese Patent No. 3373516 (referred to as '516) relates to the impact improvement of a thermoplastic material. '516 describes a thermoplastic olefin polymer indicating a good low temperature impact performance by formulating a linear ethylene α-olefin polymer with polypropylene. Table 9 indicating evaluations of Examples lists Gardner drop impact strengths (m·kg) at −20° C., −30° C. and −40° C. The linear ethylene α-olefin polymer has a linear molecular structure and a narrow molecular weight distribution width, and thus is excellent in low temperature impact performance, whereas it tends to decreases in rigidity. The highest tensile elastic modulus (MPa) is 1556 in Table 15. This value is allowable for a structure; however, it is a value for an injection molding sample. Even in the whole specification of '516, blow molding properties are not depicted.
Japanese Patent No. 3470337 (referred to as '337) relates to a composition composed of a propylene homopolymer and propylene-ethylene copolymer. '337 describes an improvement in impact resistance at row temperatures. Tables 13 and 14 of Examples depict examples of hollow molded articles. However, the molded articles have a cornice structure with flexibility, and a propylene-ethylene copolymer is a random copolymer, and thus is not usable as a structure that requires rigidity.
Japanese Patent No. 2730879 (referred to as '879) relates to a resin composition for blow molding, produced by formulation of polypropylene resin (block copolymer or homopolymer), high density polyethylene and talc. The viscosity of high density polyethylene is set in a specified range so as not to spoil blow molding properties even if talc is formulated for the improvement of rigidity. A bumper beam of a molded article described in '879 has a bending elastic modulus of 18700 to 22100 kg/cm3 according to the description of an Example and thus has rigidity as a structure. However, there is no description of low-temperature impact properties as a composition or molded article.
Japanese Patent Laid-Open (JP-A) No. 10-235720 relates to a blow molded surface panel for use in an automobile cargo floor panel and the like. In a structure as described in JP-A-10-235720, a blow molded panel made of a propylene resin composition is not found, which is excellent in rigidity, impact resistance and blow molding properties as well as drop impact properties at low temperatures, and is balanced among a variety of characteristics. Thus, the development of a material that is more excellent in characteristics is desired.
JP-A-2003-291935 describes a multi-layered blow vessel comprised of a composition made by blending a propylene random copolymer and a propylene block copolymer, which is well balanced among heat resistance, impact resistance and transparency in order to meet impact resistance at low temperatures. In JP-A-2003-291935, the article is improved in impact resistance at low temperatures by formulation of a propylene block copolymer. However, a propylene random copolymer is present as a formulation component, and thus the vessel is unsuitable for applications that require rigidity.
Propylene-ethylene block copolymers are excellently balanced among rigidity, impact resistance and heat resistance. However, individual resin characteristics are desired to be further improved responding to performance improvement of each product. Measurements for required characteristics include improvements by means of molecular weight, molecular weight distribution, steric regularity, additive and the like. Among resin characteristics, further improvements are desired in impact resistance at low temperatures for automobile parts and the like. An improvement in impact resistance at low temperature would be considered by means of improvement of the molecular weight distribution and the like of polymer itself. However, for compensation, these pose the problem of destroying balances among various physical properties such as deterioration of blow molding properties.
Improvements in impact resistance at low temperatures by means of additive are known in many documents. It is useful to add a rubber material such as ethylene/propylene rubber. However, this measure presents a problem. In other words, it causes the problem of lowering rigidity by formulation. It is also known to blend an inorganic filler such as talc for the purpose of maintaining rigidity. However, mixing of talc could further worsen mechanical physical properties due to inhomogeneous dispersion. Mixing of talc in a larger amount tends to be a factor of inhibiting blow molding properties. For this reason, it is known to formulate high density polyethylene for improving blow molding properties. High density polyethylene is comparatively inexpensively available, so that formulation is useful and the formulation improves impact resistance and rigidity at room temperature. However, improvements in impact resistance and rigidity at low temperatures cannot be expected. That is, it is difficult to simultaneously achieve rigidity, impact resistance, cold resistance, heat resistance, and blow molding properties. Thus, a composition of meeting these properties is desired.
As discussed above, formulation of high density polyethylene improves blow molding properties of a polypropylene block copolymer to some extent. However, it is assumed to be difficult to attain various demand characteristics with good balance. For measures of these kinds, some proposals are made as described in the “Background Art”; however, these are not satisfied yet.
The present inventor carried out blow molding on a variety of materials of such conventionally known compositions of a polypropylene block copolymer and high density polyethylene and evaluated them. As a result, the inventor could find that it is difficult to obtain a molded article in which the rigidity and impact resistance are maintained at room temperature, the blow molding properties are improved, and further the impact resistance is maintained at a low temperature of about −30° C.